Uhm, what is all that jibberjabber...
There are a couple of new things here that are very different compared to how you would configure this in your crontab.

First, you have to have a service defined for a timer, otherwise it just won't work, if you have documentation saying otherwise, please let me know.

Second, the service is defined as a oneshot service, this is important for the DuckDns updater since it should not be running all the time. And the timer will invoke it every time it elapses, and once done there is nothing more for it to do.

Third, the format for the timer config, OnBootSec=5min this ensures that after the system boots, it will wait for 5min before issuing the first run of the timer. OnCalendar=*:0/5 means that we want the timer to be invoked every 5min. For a nice detailed walk through on how timers works under systemd, see https://wiki.archlinux.org/index.php/Systemd/Timers

And finally, you will have to replace [YOUR_DOMAIN_NAME_HERE] and [YOUR_TOKEN_HERE] with your own domain and token from the DuckDNS dashboard.

Once you have created the above files, you will need to start the service part of it to ensure that it works as expected.

Issue:

systemctl start duckdns.service

Validate that your timer has been registered:

systemctl list-timers

You should see some output from the list-timers command similar to this: